While I’m not one of the developers, I found a sweet spot for my images to look good and load fast – I export them out of Lightroom at 72dpi, 2100px for the long side, Quality 80, sRGB, no sharpening, delete EXIF except for Copyright Info. It’s unlikely that any photo will go above the 1MB mark – while retaining a good on-screen quality even on larger screens.
The recommended image size is 2100px by the bigger side. Basically the theme will resize any bigger images to several sizes for use in different places. The biggest of them being 2100px by the bigger side.